The present invention relates to a method of making an optical device. The present invention also relates to a filter which, for instance, may be used as a spectral filter. Such optical devices and filters may be used in liquid crystals, displays, interference filters, color filters, holography, optical and electronic measurement and sensing systems, and are suitable for high flux applications.
R. Maurer et al, xe2x80x9cPolarising Color Filters made from Cholesteric LC Siliconesxe2x80x9d, SID digest, pp 110-112, 1990 discloses the use of cholesteric liquid crystal polymers for color filters. Such filters reflect a limited bandwidth of light of one circular polarisation and transmit other light. Stacking of cholesteric color filters makes it possible to obtain elements which transmit only a narrow band of wavelength in the visible spectrum. This technique may be used to create transmissive primary color filters for red, green and blue suitable for use in displays. Cholesteric films may be patterned lithographically by exposing regions of the films to ultraviolet (UV) irradiation at different temperatures, for instance as disclosed in U.S. Pat. No. 4, 637,896.
EP 606,940 discloses a technique for making circular polarisers by increasing the reflection bandwidth of cholesteric films from about 50 nanometres to about 300 nanometres. In particular, a combination of diffusion and a UV intensity profile is used to increase the polariser bandwidth. EP 720,041 discloses a backlight for a liquid crystal device (LCD) using patterned cholesteric transmissive color filters. Light reflected by the filters is recirculated and returned to the display so as to improve the efficiency of illumination. It is desirable that such color filters work correctly for a large range of angles of incidence, for instance so as to improve the viewing angle of a display. D. J. Broer, xe2x80x9cMolecular Architectures in Thin Plastic Films by In-Situ Photopolymerisation of Reactive Liquid Crystalsxe2x80x9d, SID 95 digest, pp 165-168, 1995, G. M. Davis xe2x80x9cLiquid Crystal Polymer Thin Film Anisotropic Optical Componentsxe2x80x9d, Sharp Technical Journal, pp 22-25, vol. 63, December 1995 and U.S. Pat. No. 4,983,479 disclose techniques for providing three dimensional control of molecular order in polymer films, for instance using photoinitiated polymerisation or cross-linking of liquid crystal molecules.
It is well known that the wavelength reflected by a single pitch cholesteric film varies with angle of incidence according to:
xcex(xcex1)=xcex0 cos [sin xe2x88x921(2sinxcex1/n)]
where xcex0 is the central wavelength for normal incidence, xcex(xcex1) is the centre wavelength for light incident at an angle xcex1, and n/2 is the average refractive index (n0+ne)/2, where n0 and ne are the ordinary and extraordinary indices of the cholesteric material respectively. It is also known that the polarisation state of the reflected and transmitted light has a complex dependence on wavelength and angle of incidence of the illuminating light, for instance as disclosed in V. A. Belyakov et al, xe2x80x9cOptics of Cholesteric Liquid Crystalsxe2x80x9d, Sov. Phys. Usp. 22(2), pp 63-88, February 1979 and G. Joly et al, xe2x80x9cOptical Properties of the Interface between a Twisted Liquid Crystal and an Isotropic Transparent Mediumxe2x80x9d, J. Optics, vol. 25, pp 177-186, 1994. Such variations and dependencies are undesirable for many applications, for instance of color filters, where behaviour substantially independent of angle of incidence is desired. For graded pitch cholesteric devices in which the cholesteric pitch varies so as to increase the width of reflection bandwidth, the angular dependence is more complex. L. E. Hajdo et al, J. Opt. Soc. Am. vol. 69, no. 7, July 1979 xe2x80x9cTheory of Light Reflection by Cholesteric Liquid Crystals Possessing a Pitch Gradientxe2x80x9d deals only with light incident normally on the cholesteric layer.
GB-A-2 166 755 discloses a method of selectively polymerising a cholesteric liquid crystal monomer by masking the liquid crystal and curing the un-masked areas by irradiation with ultra violet light. The whole liquid crystal is irradiated. However, 3-dimensional effects arise because the liquid crystal near the surface of the irradiated areas will not be completely polymerised, since oxygen will inhibit the polymerisation. If the irradiation is carried out in air, therefore, the regions of the liquid crystal near the surface will have different properties from the polymer in the internal regions of the liquid crystal. This document does not disclose irradiating the liquid crystal in such a way that the depth to which the liquid crystal is irradiated can be controlled.
GB-A-2 132 623 discloses the production of structures whose properties have 2-dimensional variationsxe2x80x94they vary over the surface area of the structure. A liquid crystal layer is irradiated through a first mask under a first set of conditions. The mask is then removed and the non-polymerised areas are subsequently irradiated under different conditions. This will lead to a structure in which the properties vary over the area of the structure but are constant over the depth of the structure.
EP-A-0 154 953 provides an optical filter having two separate polymer films. The first film is polymerised under one set of conditions, and the second film is polymerised under different conditions.
EP-A-0 397 263 discloses a method of manufacturing a polariser by irradiating a liquid crystal monomer.
Multilayer cholesteric filters are disclosed in EP-A-0 720 041, U.S. Pat. No. 5,548, 422, U.S. Pat. No. 4,726,663, JP-A-61 032 801 and xe2x80x9cIBM Technical Disclosure Bulletinxe2x80x9d Vol 15, No 8, pp 2538-2539. These documents primarily relate to the case of normal incidence.
According to a first aspect of the invention, a filter is provided for passing optical radiation of a first circular polarisation in a first waveband incident on an input surface. The filter includes a first layer having a first cholesteric reflector for reflecting normally incident radiation of the first circular polarisation in a second waveband below the first waveband. The filter further includes a second layer which has a second cholesteric reflector for reflecting normally incident radiation transmitted by the first reflector of the first circular polarisation in a third waveband below the second waveband. In addition, the filter includes a third layer which has a third cholesteric reflector for reflecting normally incident radiation transmitted by the first and second reflectors of a second circular polarisation substantially orthogonal to the first circular polarisation in a fourth waveband at least partially between the first and second wavebands. The first and second layers have a total birefringence for non-normally incident radiation in the second waveband such that non-normally incident radiation in the second waveband is at least partially converted from the first polarisation to the second polarisation such that both normally incident and non-normally incident radiation of the first circular polarisation in the first waveband are passed.
According to a second aspect of the invention, a filter is provided for passing optical radiation of a first circular polarisation in a second waveband incident on an input surface. The filter includes a first layer which has a first cholesteric reflector for reflecting normally incident radiation of the first circular polarisation in a fifth waveband, and a second layer which includes a second cholesteric reflector for reflecting normally incident radiation transmitted by the first reflector of the first circular polarisation in a third waveband below the second waveband. The filter further includes a third layer which has a third cholesteric reflector for reflecting normally incident radiation transmitted by the first and second reflectors of the first circular polarisation in a first waveband between the fifth and second wavebands, and a fourth layer which has a fourth cholesteric reflector for reflecting normally incident radiation transmitted by the first, second and third reflectors of a second circular polarisation substantially orthogonal to the first circular polarisation in the second waveband. The first and second layers have a total birefringence for non-normally incident radiation in the second waveband such that non-normally incident radiation in the second waveband is at least partially converted from the first circular polarisation to the second circular polarisation. The first, second and third layers have a total birefringence for non-normally incident radiation in the third waveband such that non-normally incident radiation in the third waveband is at least partially converted from the first circular polarisation to the second circular polarisation such that both normally incident and non-normally incident radiation of the first circular polarisation in the second waveband are passed.
According to a third aspect of the invention, a filter is provided for passing optical radiation of a first circular polarisation in a third waveband incident on an input surface. The filter includes a first layer which has a first cholesteric reflector for reflecting normally incident radiation of the first circular polarisation in a fifth waveband above the third waveband, a second layer which has a second cholesteric reflector for reflecting normally incident radiation transmitted by the first reflector of the first circular polarisation in a first waveband between the fifth and third wavebands, and a third layer which has a third cholesteric reflector for reflecting normally incident radiation transmitted by the first and second reflectors of the first circular polarisation in a second waveband between the first and third wavebands. The first and second layers have a total birefringence for non-normally incident radiation in the third waveband such that the non-normally incident radiation in the third waveband is at least partially converted to a second circular polarisation substantially orthogonal to the first circular polarisation such that both normally incident and non-normally incident radiation of the first circular polarisation in the third waveband are passed.
The present invention thus relates to a filter and method which allows three-dimensional structures to be formed in a polymer film as a single element or device. For instance, this method may be used to form the various filters, although such filters may less-advantageously be formed by other techniques. The structure of the film may be varied with depth to achieve a desired angular response in a cholesteric spectral filter or other device. The anisotroptic nature of liquid crystal molecules, the ability to control the orientation of these molecules, for instance by surface effects or application of electric or magnetic fields, and the ability of UV light to fix the orientation enable complex three dimensional structures to be created. For instance, single film notch filters and single film RGB transmissive color filters may be formed.
The angular behaviour of devices such as filters may be controlled using these techniques. For instance, it is possible to provide spectral filters which retain their performance over a wider angular range of incidence and emergence than for known devices.
Another advantage of such cholesteric color filters is that they may be used in systems with a large optical flux, such as projector systems. In particular, because unwanted light is reflected rather than being absorbed, the filters are subjected to less thermal stress. Thus, improved color stability and operating life may be achieved.